Squeeze film dampers have been used for years to add damping to rotor-bearing systems for vibration attenuation. In such systems, a thin oil film between the journal bearing housing and the bearing case provides damping by allowing the bearing housing to bounce around in the bearing case or adapter ring (hereinafter collectively referred to as “bearing case”) within the oil film. The squeeze effect on the oil produces the damping.
One challenging aspect of squeeze film damper design concerns centering the bearing housing in the bearing case. Elastomer O-rings are often used for this purpose. O-ring grooves provided in the bearing housing are offset so that the housing is high in the bearing case. When the rotor is set in the bearing, the rotor gravity load forces the bearing housing down in the bearing case. If the system is designed correctly, the bearing housing ends up being centered in the bearing case with the rotor installed.
There are numerous problems associated with using O-rings in this manner. One such problem relates to the fact that the stiffness of the O-rings is highly nonlinear, making it difficult to chose the proper diameter and thus stiffness for bearing housing centering. Another problem is that the O-rings deteriorate over time, losing their stiffness thereby allowing the bearing housing to drop down in the bearing case, reducing the effectiveness of the damper. A further problem is that, particularly for rotors that weigh over 3,000 pounds, it is difficult to find an o-ring that will provide sufficient stiffness to counteract large rotor weight.
In order to address these problems associated with using o-rings for centering the bearing housing in the bearing case, mechanical centering devices have been developed. One traditional example of such a mechanical device employs one or more arc springs for centering. However, such systems are disadvantaged in that designing the arc spring(s) resulting in appropriate properties for the each particular bearing configuration is difficult and labor-intensive.
U.S. Pat. No. 5,613,781 to Kuzdzal attempts to address these problems. The Kuzdzal patent discloses a damper film bearing assembly for supporting a rotatable shaft within an annular cavity formed in a housing. A fluid film damper mechanism acts between the annular outer surface of a bearing member and the outer wall of the cavity for damping radial movement of the bearing member within the cavity. A resiliently adjustable dead weight spring support system acts between the housing and the bearing member to support the dead weight of the shaft within a vertically centered position within the central bore so that the fluid film damper mechanism functions to maintain the shaft centered within the bore when vibrations occur during rotation of the shaft. The spring support system includes a bolt which engages the bearing member, which bolt is biased away from the bearing member by a support spring (i.e., plurality of Belleville washers) in order to provide an upwardly directed force to counterbalance the weight of the shaft and the bearing and thereby resiliently urge the bearing member and the shaft upwardly into a centered position within the central bore.
While the device disclosed in the Kuzdzal patent may obviate some of the problems associated with the earlier prior art, it still suffers from a number of disadvantages of its own. The Kuzdzal device is cumbersome and requires machining modifications to the bearing case. The bearing case is a difficult piece to machine as it is large, heavy, cumbersome and difficult to remove from the machine. For a retrofit, customers are reluctant to make a change that requires bearing case modifications. For the original equipment manufacture with a new machine, any changes to the bearing case would require deviation to a standard part, issuing new drawings and additional time and thus money spent on machining. Customers are often reluctant to do this.
Another disadvantage of the Kuzdzal device is that it would need to be assembled with the bearing case to set and check the pre-load of (i.e., the proper tightening of) the disc spring bolt and thus the proper compression of the disc springs to properly support the rotor. A further disadvantage of the Kuzdzal device is that in order to remove the bearing from the bearing case, it is necessary to disengage the disc spring bolts. As such, the pre-load must be re-set upon re-assembly, which is time and labor intensive.
What is desired, therefore, is a centering device for squeeze film dampers which is relatively easy, and not labor intensive, to design and adapt to the configurations of particular bearings, which is not prone to losing its effectiveness over time, which provides sufficient stiffness to counteract large rotor weight, which is not cumbersome and does not require machining modifications to the bearing case, and which does not require that the pre-load be set upon assembly and re-set upon each re-assembly.